The lifetimes for spontaneous emission from the X 3Σ−(v=1) and a 1Δ states of CH−

The radiative decay of excited CH− trapped in a radio frequency ion trap was measured, and the total excited state population was probed by observing the depletion of trapped CH− caused by photodetachment at 1.16 eV, below the expected electron affinity for the ground 3Σ− state. The signal decayed biexponentially with time. We assigned the long lived state (lifetime 5.9+0.8, −0.6 s) as the metastable a 1Δ state previously identified in the photoelectron spectrum. The fast decay, with a lifetime of 1.75±0.15 ms, was attributed to the first excited vibrational level of the ground 3Σ− state, in good agreement with a theoretical result by Manz, Zilch, Rosmus, and Werner. These results support the electron affinity of 1.238 eV for CH− obtained by Kasdan, Herbst, and Lineberger from photoelectron spectroscopy, and contradict the value of 0.74 eV determined by Feldmann from photodetachment spectroscopy.